Electromagnetic clutches typically are used to transmit power from a lawn mower engine to two or more cutting blades mounted on spindles under a mower deck. The electromagnetic clutches are designed to engage very quickly to minimize clutch plate wear. However, a disadvantage of the sudden engagement is that engine speed may droop. As a result, the mower deck may vibrate and shake until the rotational speed of the blades and spindles increases sufficiently. Until the blades reach the desired speed, the belt or belts may slip on the pulleys mounted on the drive shaft and blade spindles.
Electromagnetic clutches for mower blades may be engaged by actuating a push button or toggle PTO switch. If the operator notices significant engine speed droop when actuating the PTO switch, he or she may then turn off the switch to disengage the clutch before the engine stalls, either before or shortly after clutch lockup. If the operator repeatedly flips the PTO switch to attempt re-engaging the clutch to mate up the engine to a stalled load, the useful life of the clutch will be greatly reduced.
A stalled load, or stall condition, may be a mower blade that is stuck against an obstruction on the ground under the mower deck, or a mower blade that encounters high resistance rotating through a build-up of grass clippings or debris on the undersurface of the deck. These and other stall conditions can significantly slow or stop rotation of rotary cutting blades. Attempts to engage and re-engage the mower blade clutch while a stall condition exists increases clutch wear.
In the past, attempts to address the mower blade clutch engagement problem have not been very effective or economical. One approach is to size the electromagnetic clutch with sufficient capacity to stall the engine without slipping the clutch, requiring a more costly clutch assembly. Another approach is to provide a clutch that allows the belt to slip significantly before the engine stalls. However, the belt may wear excessively before the operator notices the belt slipping or smells the belt burning, and then uses the PTO switch to disengage the clutch.
Soft engagement devices also exist for electromagnetic clutches to reduce sudden clutch engagement, by modulating the voltage applied to the electromagnetic coils of the clutch. The electronic modules allow the clutch plates to slip for a period, while reducing the engine droop and other undesirable characteristics of the hard start. However, the clutch may be susceptible to wear during use of a soft engagement device.
A system is needed to reduce wear to an electromagnetic clutch for transmission of power from a lawn mower engine to mower blades, and to prevent attempted repeated engagement of the clutch while a stall condition exists.